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Reptile diversification began –during the Mississippian Period –with the evolution of the first animals to lay amniotic eggs From this basic stock of so-called stem reptiles –all other reptiles, as well as birds and mammals, evolved The Diversification of Reptiles

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Relationships among fossil and living reptiles and birds Reptiles and Birds

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Reptiles known as archosaurs archo meaning "ruling" and sauros meaning "lizard” –include crocodiles, pterosaurs (flying reptiles), dinosaurs, and the ancestors of birds Including such diverse animals –in a single group implies –that they share a common ancestor –and indeed they possess several characteristics that unite them Archosaurs and the Origin of Dinosaurs

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All dinosaurs possess –a number of shared characteristics, –yet differ enough for us to recognize two distinct orders the Saurischia and Ornithischia A distinctive pelvic structure characterizes each order –3 bones in pelvis: illium, ischium, and pubis –Saurischian: pubis points down –Orinischian: pubis points back Dinosaurs Orders

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The saurischians, –include two distinct groups –known as theropods and sauropods All theropods –were carnivorous bipeds –ranging in size from tiny Compsognathus –to giants such as Tyrannosaurus and similar species that might have weighed as much as 7 or 8 metric tons Saurischian Dinosaurs

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Cladogram showing dinosaur relationships –showing Pelvises of ornithischians and saurischians –Among the several subgroups of dinosaurs Dinosaur Cladogram theropods were carnivores all others were herbivores

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Therapods

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Sauropods

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Dinosaur Cladogram

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Compsognathus weighed only 2 or 3 kg –Bones found within its ribcage indicate it ate lizards Small Theropod Dinosaur

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The skull of Tyrannosaurus, –another theropod, –measured more than 1 m long Tyrannosaurus

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Included among the sauropods –are the truly giant, quadrupedal herbivorous dinosaurs –Apatosaurus, Diplodocus, and Brachiosaurus, the largest known land-animals of any kind Brachiosaurus, a giant even by sauropod standards, –weighed as much as 75 metric tons, –partial remains indicate that even larger sauropods may have existed Sauropods

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Were dinosaurs endotherms warm-blooded : generates internal heat to maintain body temperature –like today's mammals and birds, or were they ectotherms cold-blooded: relies on the environment + behavior to regulate body temperature –as are all of today's reptiles? At some point between dinosaur and bird, became endothermic. When? Warm-Blooded Dinosaurs?

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Brain size correlates with endothermy in modern mammals and reptiles –Some dinosaurs had more brain per body than modern reptiles –Problems: how to measure brain size? Value of correlative data? Evidence for Endothermy: Large Brain

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More compelling evidence for theropod endothermy –comes from their probable relationship to birds –recent discoveries in China of dinosaurs with feathers or a feather-like covering Today, only endotherms have hair, fur, or feathers for insulation Evidence for Endothermy: Insulation

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Evidence for Endothermy: Dinosaurs present at high latitudes –Problem: Cretaceous Interior Seaway Predator/prey relationship –Endotherms eat more therefore an ecosystem can support fewer of them –Fossil record suggests many herbivores, fewer carnivores

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Evidence for Endothermy: dominance over mammals Endotherms can move fast for long periods of time Ectotherms can only do short bursts Other possible explanations for dinosaurian head start over mammals?

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Mammals Mammals have hair, make milk, have 3 ear bones, and are warm-blooded Marsupials and placentals give birth to live babies Monotremes lay leathery eggs Some have very short pregnancies and carry immature babies in pouches (all marsupials and some monotremes) Tree of life Web Project

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Mammals Evolved from cynodonts in Late Triassic Strong fossil record of transition: Bone structure (jaw and ear) Many types of teeth Only 2 sets of teeth Teeth meet for grinding Skin instead of scales Remained mouse-sized for 150 m.y.

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One proposal has become popular since 1980 –based on a discovery in Italy –2.5-cm-thick clay layer at the Cretaceous-Tertiary boundary with a remarkably high concentration of iridium Worldwide iridium layer now known What caused the K/T extinction?

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Closeup view of the boundary clay in the Raton Basin, New Mexico Boundary Clay

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Deep sea core Recovered by Joides Resolution in 1997 Boundary Clay

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Significance of the iridium anomaly –iridium is rare in crustal rocks –found in much higher concentrations in some meteorites –May also result from expoure to supernovas (but supernovas also create isotopic anomalies not present at K/T) Accordingly, some investigators propose –meteorite impact explains the anomaly –meteorite perhaps 10 km in diameter –impact set in motion a chain of events leading to extinctions Iridium Anomaly

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Centered on the town of Chicxulub, Yucatan Peninsula of Mexico The 180-km diameter structure lies beneath layers of sedimentary rock and appears to be the right age Impact Site

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Shocked quartz tektites, small pieces of rock that were melted during the proposed impact and hurled into the atmosphere Nearby in the Carribean: tsunami deposits Evidence at Chicxulub

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The moment of impact 65 million years ago near what is now the Yucatan Peninsula and the Chicxulub crater, a few days later. Note the inner ring. What happened?

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According to the impact hypothesis –60 times the mass of the meteorite was blasted from the crust high into the atmosphere –heat generated at impact started raging forest fires that added more particulate matter to the atmosphere Sunlight was blocked for several months –caused a temporary cessation of photosynthesis –food chains collapsed and extinctions followed Impact Consequences

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With sunlight greatly diminished, Earth's surface temperatures were drastically reduced, adding to the biologic stress Another proposed consequence of an impact is that sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ) resulted from vaporized rock and atmospheric gases Both would have contributed to strongly acid rain that might have had devastating effects on vegetation and marine organisms Acid Rain